1. Field of the Invention
This invention relates to processes for the hydrogenation of aromatic amines to corresponding alicyclic amines and more particularly pertains to the preparation of mixed isomeric methylene-bridged polycyclohexylpolyamines in high yields by the hydrogenation of mixed isomeric methylene-bridged polyphenyl polyamine feed mixtures. Isomeric methylene-bridged polycyclohexylpolyamines are useful in the preparation of polymeric materials such as polyamindes, polyisocyanates, and the like.
2. Description of the Prior Art
It is well-known that certain methylene-bridged polycyclohexylpolyamines, the dimers of which are sometimes referred to as di- or bis-(aminocyclohexyl)methanes, can be prepared by the hydrogenation of certain methylene-bridged polyphenyl polyamines utilizing certain noble metal catalysts. For example, U.S. Pat. No. 2,606,927 to Barkdoll, et al, 1952, describes a process wherein hydrogen is reacted with a dicarbocyclic diamine containing at least one benzene nucleus and having carbocyclic nuclei separated by at least one acyclic carbon atom, at a temperature of 225.degree.-300.degree.C. under a pressure of at least 500 atmospheres in the presence of ammonia and a nickel or cobalt hydrogenation catalyst. However, the described process has the distinct disadvantage of producing very low yields of desired products, e.g., below about 52.4%, and requires long reaction times approaching two to three hours. The overall combination of high temperature, high pressure and long reaction times also apparently increase the yield of undesirable solid by-products which are relatively useless.
Due to these disadvantages and others, the utilization of catalysts containing metallic ruthenium has become preferred in the art in such hydrogenation processes as described in U.S. Pat. Nos. 2,606,924; 2,606,925; 2,494,563; 3,347,917; 3,676,495; and others. U.S. Pat. No. 2,606,924 to Whitman (1952) and U.S. Pat. No. 2,494,563 to Kirk, et al. (1950) describe processes for the preparation of certain stereoisomeric bis (4-aminocyclohexyl)methanes by the hydrogenation of bis(4-aminophenyl)methane with a ruthenium metal catalyst in the presence or absence of volatile organic solvent. More particularly, Whitman (2,606,924) discloses that steroisomeric bis(4-aminocyclohexyl)methane mixtures liquid at normal temperatures can be produced by the hydrogenation of bis(4-aminophenyl)methane at a temperature of 75.degree.-125.degree.C. and a pressure of between 100-15,000 psi in the presence of a ruthenium catalyst. Kirk, et al. (2,494,563) teach that the hydrogenation of bis(4-aminophenyl)methane in the presence of a ruthenium catalyst at temperatures between 140.degree.-250.degree.C. and pressures above 500 psi, e.g., 1,500-3,500 psi, results in a reaction product which, upon distillation and redistillation in vacuo, provides stereoisomeric bis (4-aminocyclohexyl)methane mixtures having a freezing point above 40.degree.C.
U.S. Pat. No. 2,606,925 to Whitman (1952) and U.S. Pat. No. 3,347,917 to Arthur (1967) both describe the preparation of steroisomeric di(para-aminocyclohexyl)methane rich in the trans,trans-stereoisomer by the hydrogenation of di(para-aminophenyl)methane in the presence of a ruthenium catalyst and ammonia. More particularly, Arthur (3,347,917) teaches that the desired product, liquid at room temperature, can be prepared in high yields above 90% in short reaction times by hydrogenating di(para-aminophenyl)methane at the temperatures of 180.degree.-300.degree.C. and pressures above 500 psi in the presence of 0.01% to 10% metallic ruthenium and 1% to 100% of ammonia, both based on the weight of the diamine feed. Patentee also discloses that the di(para-aminophenyl) methane feed prepared by conventional procedures, or obtained from commercial sources, can contain up to several percent impurities which are principally the ortho,para'-methylenedianiline. However, as more particularly discussed hereafter, it has been found that the presence of such isomeric impurities in the feed results in deactivation of the ruthenium catalyst and rapid decline in hydrogenation efficiency.
Further, U.S. Pat. No. 3,676,495 (1972) to Guenther describes the preparation of the triamine 2,4-bis (aminocyclohexylmethyl)cyclohexylamine by hydrogenation of 2,4-bis(p-aminobenzyl)aniline in the presence of a ruthenium metal catalyst and ammonia as described in U.S. Pat. No. 2,606,925, supra.
As evidenced by the above references, hydrogenation of methylene-bridged polyphenyl polyamines in the presence of ruthenium catalysts normally includes employment of substantially pure feeds, e.g., 4,4'-methylenedianiline, substantially free from other isomers such as 2,4'- and 2,2'-methylenedianiline isomers. These feeds are prepared by the well-known condensation reaction of aniline and formaldehyde in the presence of a strong mineral catalyst, such as HCl, or a silica-alumina catalyst. The well-known condensation reaction generally produces a mixed isomereic condensation product containing methylenedianiline isomers and higher functionality polyphenyl polyamines and their isomers. As known, the amounts of particular isomers and higher functionality polyphenyl polyamines can be controlled to some extent by control of condensation reaction process conditions. Yet, substantially pure feeds have heretofore usually been obtained by further isolating a particularly desired isomer from the condensation reaction product such as by fractional distillation.
From an economic standpoint, it would be desirable to prepare methylene-bridged polycyclohexylpolyamine products by the hydrogenation of the mixed isomeric condensation reaction products without the requirement of isolating substantially pure feeds. It is well-known that mixed isomeric methylene-bridged polyphenyl polyamines can be hydrogenated to the corresponding mixed isomeric methylene-bridged polycyclohexylpolyamines by the employment of ruthenium-containing hydrogenation catalysts. However, such hydrogenation of methylene-bridged polyphenyl polyamines suffers from the disadvanatage of rapidly deactivating ruthenium-containing hydrogenation catalysts. Apparently, these mixed isomeric polyamines "poison" ruthenium catalysts. In continuous hydrogenation systems, the rapid deactivation of the ruthenium catalyst results in partial and eventual cessation of hydrogenation, which further results in low yields of hydrogenated product.
Such catalyst deactivation requires that the catalyst be frequently replaced with fresh active catalyst. Moreover, such deactivation can result in partial loss of the ruthenium metal or catalyst activity. Ruthenium-containing hydrogenation catalysts are very expensive and, therefore, in view of these disadvantages, it has heretofore been more economical to employ substantially pure polyamine feeds in hydrogenation processes utilizing ruthenium catalysts.
We have now discovered a process for hydrogenating mixed isomeric polyphenyl polyamine mixtures employing a ruthenium-containing hydrogenation catalyst that does not result in catalyst deactivation or otherwise adversely affect the catalyst activity. In our process, mixed isomeric polyphenyl polyamine feed mixtures are pretreated by initially contacting the feed mixtures with a nickel-containing hydrogenation catalyst. Surprisingly, the pretreatment in accordance with the present invention apparently removes catalyst poisons in the feed or otherwise prevents ruthenium catalyst deactivation and results in the production of mixed isomeric methylene-bridged polycyclohexylpolyamines in very high yields, e.g., above 90% based on the weight of the polyamine feeds.